The present application claims the benefit of foreign priority on Netherlands application 1015085, filed May 2, 2000.
The present invention relates to a dissolving device and method for dissolving a particulate solid in a stream of a supercritical or almost critical fluid.
A dissolving device of this type is known, for example, from international patent application WO 97/14843, which describes a method for dyeing a textile substrate in a supercritical fluid, such as CO2, in which a dye is dissolved. In order to dissolve a particulate dye in the fluid, this dye is placed in a dye container, for example as a fixed bed or fluidized bed between perforated plates, through which a stream of the supercritical fluid is passed. A solution of dye and supercritical fluid which is formed in this way is then passed over and through the textile substrate which is to be dyed, the dye being deposited on the substrate. The substrate is situated in a pressure vessel which, together with the dye container and other necessary components such as a circulation pump, is accommodated in a loop of lines.
In practice, however, it has been found that with this design of the dye container and under the pressure and temperature conditions used, sintering of the dye may occur, which reduces the solubility of the dye in the supercritical fluid. Furthermore, it is necessary to prevent the fluid stream from containing dye particles of relatively large dimensions (for example particles of 30 micrometers and larger), since these dye particles become deposited on the substrate and reduce the evenness of and cause dye spots on the substrate. To this end, the abovementioned perforated plates also function as filters.
Furthermore, the conventional dyeing containers have the significant drawback that flow through these containers causes a high pressure drop. This pressure drop limits the amount of supercritical fluid which can be pumped through the dyeing container. However, since the quantity of supercritical fluid per unit time is partially responsible for determining the dissolution rate and therefore the rate of the dyeing process, this represents a significant restriction. In practice, this restriction can be overcome by installing a pump with a high working head. Another possibility for eliminating this restriction is the use of a dyeing container of significantly larger dimensions. However, both the solutions described above entail additional costs.
Thus there is a need to provide a dissolving device for dissolving a particulate solid in a supercritical or almost critical fluid in which the pressure drop across the dissolving device is low and in which scarcely any undissolved solid particles are entrained in the fluid stream.
According to the invention, to this end the dissolving device comprises a circulation loop, comprising a feed for feeding a feed stream of the supercritical or almost critical fluid, a cyclone, which is in communication with the feed and has a principal discharge for discharging a principal discharge stream of a solution of the particulate solid in the supercritical or almost critical fluid and has an auxiliary discharge for discharging an auxiliary stream of the supercritical or almost critical fluid with solid particles dispersed therein, the auxiliary discharge being in communication with the said feed.
In the dissolving device according to the invention, the particulate solid which is to be dissolved is introduced into a circulation loop in which a cyclone is incorporated. In the turbulent flow in the cyclone, the solid particles are brought into intimate contact with the supercritical or almost critical fluid. In the cyclone, the undissolved solid particles are flung outwards by the centrifugal force and are discharged at the bottom of the cyclone via the auxiliary discharge (as is known, particles are separated according to mass in a cyclone). The principal discharge stream, which is discharged at the top of the cyclone, comprises a solution of the solid particles in the supercritical or almost critical fluid. The auxiliary stream of supercritical or almost critical fluid containing dispersed solid particles is introduced back into the feed stream of the cyclone, so that the solid particles continue to circulate until they have been sufficiently dissolved. When using a cyclone to dissolve a particulate solid in a supercritical or almost critical fluid, the pressure drop is low compared to the (dyeing) containers for dye particles according to the prior art. When using the dissolving device according to the invention in a dyeing method with a solution of dye in a supercritical or almost critical fluid, it is thus possible for this dyeing process to be carried out more quickly. Moreover, there is no need for additional investment costs on pumps with a high working head and/or dyeing containers of large dimensions, which leads to lower costs.
The dissolving device according to the invention does not have to contain additional filters, since the cyclone itself may be designed and operated in such a manner that separation according to the desired particle dimensions/mass takes place therein. An additional pressure drop caused by the filters in the devices according to the prior art, such as the perforated plates described above, is thus avoided.